Good news! Eisenmann have now redesigned the quad system for the North American spec 135i!

The past Eisenmann system for the 135i in 4x76mm quad configuration had to be redesigned to fit the North American 135i model. After more research, Eisenmann concluded that there is a fuel evaporator canister on the North American 135i that is not on the European model. That canister would hit the top portion of the right side outlet piping causing minor rubbing on the 135i here in North America.

With the revised version of the Eisenmann 135i 4x76mm quad exhaust there are no question about perfect fitment.

Two options are available for an exhaust manufacturer when creating a true dual exit exhaust system: you can either design a center canister with one pipe leading out to each side of the car, or you can create two canisters so that each side of the car has it's own dedicated canister and piping.

Here are photos of each type of design for an E9x M3 application, just for an example:

Dedicated piping and canister for each side of the car.

Center canister, with pipes going to each exit

The 135i does not have the available space below the car to house either of these options. If you wanted to use either of the above choices, you would have to cut out the storage well in the trunk area, weld in flat sheet metal in it's place, weld exhaust hangers, and remove the charcoal canister located in the rear of the car.

That process involves far more custom work than people are willing to accept, and Eisenmann provides a simple bolt-on solution instead. Eisenmann's piping is routed to bypass all of the offending OE equipment and direct exhaust gasses to the passenger side of the car.

I hope that explains why the "dummy pipe" compromise is made. It's technically not an ideal solution but is a better choice for most people than the thousands of dollars of labor required to do the job using two canisters.

Eisenmann's canister design uses a very different flow path and only needs the driver's side tips to create enough exhaust flow volume. The passenger side piping is large enough to allow exhaust gasses to flow through the passenger side tips for a realistic appearance, but do not need to be full sized pipes from a functionality standpoint.